This invention relates to an image forming apparatus such as a copying machine, a printer, and a fax, and in particular, a color image forming apparatus of electrophotographic type wherein a plurality of sets of charging means, image exposure means, and developing means are arranged to form a color image within one rotation of an image forming member by superposing toner images.
Heretofore, for the method to form a multi-color image, the following have been known: a color image forming apparatus wherein a number of image forming members, charging means, developing means and so forth, said number being the same as the number of colors required to form a color image, are provided to make the color image by superposing monochromatic toner images formed on each of the image forming members on a transfer material; a color image forming apparatus wherein charging, image exposure, and development are repeated for the respective colors by rotating an image forming member a plurality of times to form the color image; a color image forming apparatus wherein charging, image exposure, and development for the respective colors are sequentially done within one rotation of an image forming member to form a color image; and so forth.
However, among the above-mentioned image forming apparatus, the color image forming apparatus wherein a number of image forming members, charging means, developing means and so forth, said number being the same as the number of colors required to form a color image, are provided to make the color image by superposing monochromatic toner images formed on each of the image forming members on a transfer material has a defect that it requires a plurality of image forming members and means for transporting a transfer material to make the apparatus large-sized; and on the other hand, the color image forming apparatus wherein charging, image exposure, and development are repeated for the respective colors by rotating an image forming member a plurality of times to form the color image is made small-sized with respect to its volume, but it has the limitation that the size of the image to be formed is limited to the same or smaller size than the surface area of the image forming member.
In that regard, the color image forming apparatus wherein charging, image exposure, and development for the respective colors are sequentially done within one rotation of an image forming member to form a color image has advantages such as that it has no limitation in the size of the image and that it enables a high-speed image forming. Further, the one wherein a transparent substrate is used for the image forming member and the image forming means are disposed inside the image forming member to make the apparatus small-sized is proposed in the Japanese laid open patent H5-307307, for example.
Further, for the photosensitive layer of the image forming member to be used in a color image forming apparatus, an OPC (organic photoconductor) is generally used owing to its low cost and stability.
However, in the image forming apparatus according to the above-mentioned proposal, in the case of an OPC used for the photosensitive layer, a comparatively long time (0.05-0.2 sec) is required for the electric potential to be lowered to a low level owing to its low mobility of the electric charge carrier. Moreover, it has a large dependence of the electric potential on temperature. For this reason, different to the inorganic photosensitive materials such as selenium, it is required a time period for the electric potential to be lowered from the imagewise exposure by the imagewise exposure means to the development by the developing means. That is, since it is necessary to set the developing position at the position where the electric potential is sufficiently lowered from the imagewise exposure, the apparatus inevitably becomes large in its size.
On the other hand, when the electric potential of the charged image forming member is lowered to a level below about a half (charge elimination by light) owing to the imagewise, toner scattering may take place from the toner image previously formed on the image forming member, resulting in the problem that the inside of the apparatus is soiled by the toner scattering.
It is the first object of this invention to provide an image forming apparatus wherein the above-mentioned points of problem are improved, that is, the size is made small, and at the same time, the soil on the inside of the apparatus and the soil on the charging means by the toner scattering from the toner image owing to the lowering of the electric potential is prevented; the soil on the imagewise exposure means in the case the imagewise exposure means is provided on the outside of the image forming member is also prevented.
Further, in the color image forming apparatus according to the above-mentioned proposal, it is employed a pre-transfer exposure means for eliminating the charge on the image forming member by exposing it to a uniform light, but if an OPC is used for the image forming member, a comparatively long time (0.05-0.2 sec) is required until the electric potential is lowered owing to the low mobility of the charge carrier. Moreover, the variation of the electric potential with the variation of temperature is also large. For this reason, different to the inorganic photosensitive materials such as selenium, it is required a time period for the electric potential to be lowered within the path from the pre-transfer charging means to the transfer position by the transfer means. That is, it is necessary to dispose the transfer means at a position with an enough distance from the pre-transfer exposure means for the electric potential to be lowered sufficiently, which produces the problem that the apparatus is made large-sized.
On the other hand, when the electric potential of the charged image forming member is lowered to the same as or lower than about a half level (charge elimination by light) by the uniform exposure in the pre-transfer exposure means, the electrically attaching force of the color toner image on the image forming member to the image forming member is weakened to generate the toner scattering, which causes the problem to occur that the inside of the machine is smudged by the toner particles scattered. In addition, in the case of the apparatus wherein the image exposure means are disposed outside the image forming member, the problem that the image exposure means are smudged by the toner particles scattered is also produced.
It is the second object of this invention to provide a color image forming apparatus wherein the above-mentioned points of problem are improved, that is, the apparatus is made small-sized, and at the same time, it is prevented the smudge inside the machine brought about by the toner particle scattering from the color toner image on the image forming member, of which the attaching force is weakened by the lowering of the electric potential after the uniform exposure in the pre-transfer exposure means (charge elimination by light), and further, it is also prevented the smudge of the image exposure means brought about by the toner particle scattering in the case where the image exposure means are provided outside the image forming member.
Further, in the color image forming apparatus according to the aforesaid proposal, a pre-cleaning exposure means for eliminating the residual charge on the image forming member by exposing the image forming member to the uniform exposure light in order to make it easy the cleaning of the residual toner particles after transfer, but if an OPC is used for the image forming member, a comparatively long time (0.05-0.2 sec) is required until the electric potential is lowered owing to the low mobility of the charge carrier. Moreover, the variation of the electric potential with the variation of temperature is also large. For this reason, different to the inorganic photosensitive materials such as selenium, it is required a time period for the electric potential to be lowered within the path from the position of the uniform exposure by the pre-cleaning exposure means to the cleaning position by the cleaning blade engaging with the image forming member of the cleaning means for cleaning the residual toner particles after transfer on the image forming member. That is, it is necessary to arrange the engaging position of the cleaning blade with an enough distance from the position of the uniform exposure by the pre-cleaning exposure means for the electric potential to be lowered sufficiently, which produces the problem that the apparatus is made large-sized.
On the other hand, when the electric potential of the charged image forming member is lowered to the same as or lower than about a half level (charge elimination by light) by the uniform exposure in the pre-cleaning exposure means, the electrically attaching force of the residual toner particles after transfer on the image forming member to the image forming member is weakened to generate the toner scattering, which causes it to occur the problem that the inside of the machine is smudged by the toner particles scattered. In addition, in the case of the apparatus wherein the image exposure means are disposed outside the image forming member, the problem that the image exposure means are smudged by the toner particles scattered is also produced.
It is the third object of this invention to provide a color image forming apparatus wherein the above-mentioned points of problem are improved, that is, the apparatus is made small-sized, and at the same time, it is prevented the smudge inside the machine brought about by the toner particle scattering from the residual toner particles after transfer on the image forming member, of which the attaching force is weakened by the lowering of the electric potential after the uniform exposure in the pre-cleaning exposure means (charge elimination by light), and further, it is also prevented the smudge of the image exposure means brought about by the toner particle scattering in the case where the image exposure means are provided outside the image forming member.